At every grid point for the valid forecast hour, two probabilities are paired:

Probability of UH ≥ 75 m2/s2 over the previous 4 h (from the HREF)

Probability of environmental field(s) meeting a threshold over the previous 4 h (from the SREF; see table below)

The historical frequency of a hazard report occurring within 25 miles of that grid point and within the 4-h period for that forecast pair of probabilities is substituted as the 4-h calibrated hazard probability.

Uses the 4-h hazard probability forecasts that cover the 24-h convective day (i.e., 12z-12z). At every grid point, the cumulative sum of the 4-h probabilities and the maximum 4-h probability are paired. The historical frequency of a hazard report occurring within 25 miles of that grid point and within the 24-h period for those 4-h calibrated hazard probabilities is substituted as the 24-h calibrated hazard probability.

A distribution of STP is formed for each gridpoint from points where UH exceeds the 99.99th percentile (within each member's climatology) within a 40 km radius. The 75th percentile of STP from that distribution is then assigned to each point at each hour, and then the maximum daily STP value for each point is used to assign a probability based on the climatological frequency of a tornado given a right-moving supercell and an STP value for each ensemble member. The mean at each point is taken across the members, and then a Gaussian smoother with σ = 50 km is applied.

Using a priority list of (1 - freezing rain, 2 - snow, 3 - rain), a type is chosen if more than half of precipitating ensemble members indicate the type has occurred over the past hour; if no category reaches that threshold, then the type is labeled mix

For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that *any snow* has occurred during the period (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.

For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that *any snow* has occurred during the period (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.

For each ensemble member, the precipitation falling during each 1-hour period is assumed to be *all snow* if the member indicates that *any snow* has occurred during the period (overestimation can thus occur during mixed precipitation). Uses WPC forecast snow-to-liquid ratio (SLR) when available, or 10:1 SLR otherwise.

Using a priority list of (1 - freezing rain, 2 - snow, 3 - rain), a type is chosen if more than half of precipitating ensemble members indicate the type has occurred over the past hour; if no category reaches that threshold, then the type is labeled mix

Primarily intended for post-event verification. Outlooks are overlaid which match the plotted forecast data temporally and were issued around the time the HREF run became available.

The following tables indicate the correspondence between HREF forecast hours and overlaid outlook products (pending availability).

00z HREF

Forecast Hour

Convective

Fire

1-12

01z SWODY1

N/A

13-36

06z SWODY1

07z FWDDY1

12z HREF

Forecast Hour

Convective

Fire

1-24

13z SWODY1

17z FWDDY1

25-36

N/A

N/A

SPC Categorical

SPC Tornado

SPC Hail

SPC Wind

SPC Fire

Keyboard Shortcuts

<

- prev fcst time

>

- next fcst time

p

- peek at last run (hold down)

h

- toggle top menu

More Information

HREFv2 is an operational version of SSEO

?

The High Resolution Ensemble Forecast version 2 (HREFv2) is an operational version of the SPC Storm Scale Ensemble of Opportunity (SSEO). It was developed at the NCEP/Environmental Modeling Center (EMC) and is run daily at 00, 06, 12, and 18 UTC by NCEP Central Operations (NCO). This page displays the 00 UTC and 12 UTC cycles only, and uses a 12-h time-lagged NAM Nest member rather than a 6-h time-lagged NAM Nest member. This prototype page replaces the previous SPC SSEO web page.

HREF developed at NCEP EMC and run at NCO • Data processed and plotted at NWS SPC • Please direct questions or comments to Brett Roberts • This is a prototype page and may be subject to delays or outages